1. Field of the Invention
The present invention relates to a technique of reducing the output power difference in a polar modulation transmission apparatus.
2. Description of the Related Art
FIG. 1 shows an example of a typical transmission apparatus using a polar modulation scheme. The transmission apparatus has polar signal generation circuit 1, amplitude control circuit 2, phase modulated signal generation circuit 3 and power amplifier (hereinafter “PA”) 4. In this transmission apparatus, polar signal generation circuit 1 generates an amplitude component signal and a phase component signal of a modulated transmission signal from an input signal (i.e. the modulated transmission signal). Amplitude control circuit 2 controls the power supply voltage supplied to PA 4 based on the amplitude component signal, and phase modulated signal generation circuit 3 generates a phase modulated signal inputted to PA 4 based on the phase component signal.
This transmission apparatus secures the dynamic range of transmission power by switching between compressed mode and uncompressed mode in PA 4. Further, compressed mode may be rephrased as “saturation operation mode” and uncompressed mode as “non-saturation operation mode.”
This transmission apparatus operates PA 4 in compressed mode when high transmission power is required. On the other hand, when low transmission power is required, the transmission apparatus operates PA 4 in uncompressed mode. To be more specific, in compressed mode, the transmission apparatus controls the output power by changing the level of the power supply voltage to PA 4 according to predetermined output power. Given that the PA operates in the saturation region, this compressed mode is inherently very accurate with respect to output power.
On the other hand, in uncompressed mode, the transmission apparatus controls the output power by changing the level of the phase modulated signal inputted to PA 4 according to predetermined output power. In this uncompressed mode, given that the PA operates in the linear region, the output power is not controlled as accurately as in compressed mode.
With conventional transmission apparatuses, when compressed mode (“c-mode”) and uncompressed mode (“u-mode”) are switched in transmission power control, transmission power drift is likely to occur due to differences between the characteristics of the modes (i.e. drift due to temperature, drift due to wear, and drift due to load, etc.).
This will be explained briefly using FIG. 2. As shown in FIG. 2, the output power in compressed mode is relatively accurate, but the output power in uncompressed mode changes due to drift (i.e. drift due to temperature, drift due to wear, and drift due to load, etc.).
As shown in FIG. 2, given that the output power in uncompressed mode is likely to drift due to various factors, when compressed mode and uncompressed mode are switched, the output power in uncompressed mode is likely to be discontinuous, and, as a result, significant drift of transmission power is likely to occur.
By the way, one method of controlling transmission power accurately is to measure the actual output power of a power amplifier and perform a feedback control of output power such that this measurement value becomes equal to a set target value.
Generally, for this feedback control, the method of eliminating modulation fluctuation components resulting from transmission data from the output of the power amplifier using a low-pass filter, is employed. Then, transmission power is adjusted again based on the difference between the set target value and average transmission power from which modulation fluctuation components are eliminated.
Here, more modulated components can be eliminated by setting the time constant of the low-pass filter bigger, so that more accurate transmission power control is possible.
On the other hand, if the time constant of a low-pass filter is set bigger, the response of the low-pass filter becomes poorer, and, accompanying this, the responsivity of feedback control becomes poorer. Given that there are wireless communication standards that require completing transmission power control in a very short period, in practice, the time constant of the low-pass filter cannot be set too big.
Particularly, when the above compressed mode and uncompressed mode are switched, feedback control of a higher rate is needed, and so it becomes even more difficult to make the time constant of the low-pass filter big.